Tempered or strengthened glass is often used in consumer electronic devices, such as smart phones and tablets, due to its physical and chemical durability and toughness. In general, the durability of tempered glass and glass-ceramic substrates is increased by the increasing amount of compressive stress (CS) and depth of layer (DOL) of the glass or glass-ceramic substrate. To provide a larger CS and deepen the DOL, ion exchange processes may be used to strengthen glass or glass-ceramic substrates. In ion exchange processes, a glass or glass-ceramic substrate containing at least one smaller alkali metal cation is immersed in a salt bath containing at least one larger alkali metal cation. The smaller alkali metal cations diffuse from the glass surface into the salt bath while larger alkali metal cations from the salt bath replace the smaller cations in the surface of the glass. This substitution of larger cations for smaller cations in the glass generates a layer of compressive stress layer in the surface of the glass, thus increasing the resistance of the glass to breakage.
As the ion exchange proceeds, the salt concentration of the smaller alkali metal cations (i.e., the cations that diffuse from the glass into the salt) increases while the salt bath concentration of the larger alkali metal cations (i.e., the cations that migrate into the glass from the salt) decreases. This fluctuation in the ion concentration may cause unwanted constituents to form, which can age or “poison” the salt bath, or cause salt crystals to form and adhere to the surface of the glass. A poisoned salt bath will not produce a large CS and deep DOL in the glass substrate, as desired. Likewise, crystals on the surface of the glass can form defects—including depressions and protrusions. Dimpled, stippled glass, and glass that is not properly strengthened is not commercially desired and may be unsuitable for use in some industries